1. Field of the Invention
This invention relates to a member for charging having improved charging ability, particularly to a member for charging having improved environmental stability and giving no deleterious influence to the surface of a member to be charged.
2. Related Background Art
Heretofore, as the photoconductive material to be used in electrophotographic photosensitive member, inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc. have been known. These photoconductive materials have a number of advantages such as charging to an appropriate potential in dark place, little dissipation of charges in dark place, or rapid dissipation of charges by photoirradiation, etc, while having also on the other hand various disadvantages.
On the other hand, it has been discovered that specific organic compounds have photoconductivity. For example, organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, etc., low molecular weight organic photoconductive materials such as carbazole, anthracene, pyrazoline, oxadiazole, hydrazone, polyarylalkane, etc., and otherwise organic pigment or dyes such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene pigments, indigo dyes, thioindigo dyes or squaric acid methine dyes, etc. have been known. Particularly, since organic photoconductive materials such as organic pigments or dyes having photoconductivity can be synthesised more easily as compared with inorganic materials, and yet variation in selection of compounds exhibiting photoconductivity in appropriate wavelength region is expanded, a large number of such materials have been proposed. For example, as disclosed in U.S. Pat. Nos. 4,123,270, 4,251,613, 4,251,614, 4,256,821, 4,260,672, 4,268,596, 4,278,747, 4,293,628, etc., electrophotographic photosensitive members by use of disazopigments exhibiting photoconductivity as the charge generation substance in the photosensitive layer having functions separated into the charge generation layer and the charge transport layer have been known.
The charging process in the electrophotographic process by use of such electrophotographic photosensitive member mostly applies high voltage (DC 5-8 kV) on a metal wire to effect charging by the corona generated. However, according to such method, the surface of the photosensitive member is denatured by corona products such as ozone, NOx, etc. during corona generation, whereby image ambiguity or deterioration may be progressed, or contamination of the wire may affect the image quality, thus involving such problems as generation of image white drop-out or black streaks. Particularly, an electrophotographic photosensitive member having a photosensitive member containing an organic photoconductive material has chemical reactivity because the organic photoconductive material is an organic compound, and is susceptible to deterioration by the corona products.
On the other hand, also as the power source, the current directed toward the photosensitive member was only about 5 to 30% thereof, with most of it flowing to the shielding plate, thus being poor in efficiency as the charging means.
For compensating for such drawbacks, there have been investigated the method of direct charging by contacting a member for charging with a member to be charged such as photosensitive member as disclosed in Japanese Laid-open Patent Publications Nos. 57-178267, 56-104351, 58-40566, 58-139156, 58-150975.
In the prior art, as the member for charging to be used for direct charging, an electroconductive rubber roller having electroconductive particles such as carbon dispersed in a metal core material, or a roller coated with nylon or polyurethane as disclosed in Japanese Patent Publication No. 50-13661 have been known.
However, the electroconductive roller having electroconductive particles dispersed therein of the former is required to increase the amount of the electroconductive particles in order to retain its low resistivity, whereby the rubber hardness is increased, and further due to the hardness of the electroconductive particles dispersed on the surface, there has been the problem that the surface of the member to be charge is damaged. Particularly, in the case when the member to be charged is an electrophotographic photosensitive member having a photosensitive layer containing an organic photoconductive material, its surface hardness is extremely lower as compared with other photosensitive members, and therefore it is susceptible to damage with such electroconductive roller, whereby image defects such as streaks caused by such damage will occur. Further, there has been also involved the problem that no uniform charging can be effected due to irregularity, variance of the electroconductive particles dispersed in the electroconductive rubber roller.
On the other hand, in the case of a roller coated with nylon or polyurethane of the latter, its electrical resistance is greatly affected by the change in use environment, particularly by the change in humidity in the air. For example, under low temperature and low humidity, there has been the problem with respect to environmental stability that its volume resistivity is increased by 3 ciphers. If the member for charging is increased in resistivity, the charging ability will be lowered to effect no uniform charging, and the image density will be lowered when image formation is effected, or in the reversal developing method, black dot images in specles corresponding to charging irregularity (black spots) may be formed, while in the normal developing system white dot images (white spots) may be formed, whereby no image of high quality can be obtained in either case. Particularly in the case of nylon, there is also the problem that the photosensitive member is susceptible to damage due to its hardness.